Fiber optic pressure sensors have been topic of intense research during the last decade as they can be made very small, compact, immune to electromagnetic interference, biocompatible and can be used at elevated temperature or in a harsh chemical environment. Applications for such sensors are therefore numerous and range from combustion and jet engine controls to biomedical applications.
It is known in the prior art to provide pressure sensors at the tip of an optical fiber. For example, published international patent application no. WO 02/23148 (WILNER et al.) shows a housing made from silicon in which the end section of a lead fiber and a silicon diaphragm are fixed. The flat cleaved lead fiber end and the diaphragm form two reflective surfaces that define a Fabry-Perot interferometer. The optical signal from the lead optical fiber is split into two paths which mutually interfere. The sum of their interference is function of the distance between the fiber end and the diaphragm position, the latter being a function of pressure.
Other sensors of this type are for example described in patents and patent applications EP 1 089 062; U.S. Pat. No. 4,991,590; U.S. Pat. No. 5,381,229; U.S. Pat. No. 6,539,136; U.S. Pat. No. 5,657,405; U.S. Pat. No. 6,597,820; U.S. Pat. No. 6,820,488; U.S. Pat. No. 6,823,738; US 2004/0223679; US 2005/0041905; WO 99/45352 and WO 02/23148. (Original) The following publications may also be consulted: MacPherson W. N. et al., “Miniature fiber optic pressure sensor for turbomachinary applications”, Rev. Sci. Instr., 1999, Vol. 70 n.sup.o3, pp. 1868 1874,); Pinet E. et al. “Miniature fiber optic pressure sensor for medical applications: an opportunity for intra-aortic balloon pumping (IABP) therapy”, Proc. of SPIE, 2005, Vol. 5855, pp. 234-237. For all the solutions mentioned in these documents, the typical design produces a sensor that is larger than the diameter of the lead optic fiber used for its interrogation.
U.S. Pat. No. 4,078,432, U.S. Pat. No. 4,701,614, U.S. Pat. No. 4,918,305 and U.S. Pat. No. 5,193,129 describe small micro-binding fiber optic pressure sensors. In all cases, two optical fibers are needed; one for the probing light and the other for the detected light whose intensity is dependent on the pressure applied on the sensor.
European patent EP 0 503 812 discloses a small fiber optic pressure sensor using fluorescence-quenching, suitable for example for arterial blood pressure measurements. The size of such a sensor could be made very close to the optical fiber diameter.
There are very few known devices where the sensor diameter is equal to the diameter of the lead optical fiber. Some examples could be found in patents and patent applications EP 0 997 721, US 2004/0114848. These technologies however have their drawbacks; for example, in EP 0 997 721 the manufacturing procedure is relatively complicated and requires application of special tools and materials. In US 2004/0114848, the intrinsic Fabry-Perot sensor includes a thin film sandwiched between two fiber ends.
Other ultra-miniature pressure sensors have been reported in the literature. The paper by Cibula  onlagić “Miniature fiber-optic pressure sensor with a polymer diaphragm”, Appl. Opt. 2005, Vol. 44 no14, pp. 2736 2744, and Slovene patent SI 21242 disclose a fiber optic pressure sensor with a polymer diaphragm that is not larger than the size of the optical fiber. Papers by Abeysinghe D. C et al. (“A novel MEMS pressure sensor fabricated on an optical fiber”, IEEE Phot. Tech. Lett., 2001, Vol. 13 no9, pp. 993 995,), Abeysinghe D. C et al. (“Novel MEMS pressure and temperature sensors fabricated on optical fibers” J. Micromech. Microeng. 2002, Vol. 12, pp. 229 235,) and Totsu K. et al. (“Ultra-miniature fiber-optic pressure sensor using white light interferometry”, J. Micromech. Microeng., 2005, Vol. 15, pp. 71 75,) all disclose pressure sensors manufactured at the tip of a lead optical fiber using photolithographic patterning techniques.
Other embodiments of miniature sensors are shown in U.S. patent application no. 2005/0062979 (ZHU et al.). The miniature sensor of Zhu is designed by bonding a hollow tube to a lead fiber end-face and then bonding a diaphragm to the hollowed tube to create a Fabry-Perot interferometer. Identical configurations, employing polymer diaphragm, was also reported by Cibula E. et al. (“Miniature fiber optic pressure sensor for medical applications”, Proc. IEEE Sensors 2002, Vol. 1 no12-14 pp. 711-714,) and in Slovene patent SI 21242. One drawback of such sensors is the relative fragility of the hollow spacer, both during the manufacturing of the spacer and during its use.
There is therefore a need for a miniature optical sensor which alleviates drawbacks of the prior art.